Kansas City quarterback Alex Smith watches from the sideline near the end of Sunday’s 35-28 loss to the Denver Broncos in Kansas City, Mo.

KANSAS CITY, Mo. — The line of demarcation might as well be the Kansas City Chiefs’ bye week.

In the nine weeks prior to taking some time off, the Chiefs rolled to a 9-0 record, an unimaginable turnaround from a 2-14 finish a year ago. The defense was holding opponents to just 13.3 points per game, and the offense was playing nearly mistake-free football.

Everything has changed in the last three weeks.

After a pair of losses to the Broncos sandwiched around a heartbreaking defeat to the Chargers, the Chiefs not only have watched the AFC West lead slip away, they’ve also become the first team in NFL to win its first nine games and then lose three in a row.

It’s safe to say it is gut-check time for the suddenly stumbling Chiefs.

“I told the team this isn’t college football. This isn’t the end of the season,” coach Andy Reid said. “Now it’s important that we get ourselves back and ready to go, and finish the season like we’re capable of doing, and our guys, coaches and players, will do that.”

That’s the kind of confidence that the Chiefs exuded prior to their bye week.

It’s the kind of confidence that’s been lacking of late.

Part of the reason for the abrupt turnaround in fortunes has been the schedule, which went from one of the easiest in the NFL to one of the most difficult.

The Chiefs’ first nine games were against teams that have a combined record of 41-67, while the Broncos and Chargers have a combined record of 25-11. That stretch earlier in the season included four straight games against last-place teams, and six straight against teams third or fourth in their respective divisions. Only the Cowboys (7-5) and Eagles (7-5) have winning records.

Kansas City has swapped out the Bills, which were quarterbacked by Jeff Tuel, for the Broncos, who are led by Peyton Manning. The Texans and quarterback Case Keenum have given way to the Chargers and Philip Rivers, who is having arguably the finest year of his career.

“They’ve been coming out with game plans and they’re making plays when it counts,” Chiefs safety Kendrick Lewis said. “When the game is on the line, they’re making it count.”

That also has been true.

Six of those first nine games were decided by 10 points or fewer, and the Chiefs won all of them. The last three have also been decided by 10 points or fewer, and they’ve lost all of them.

First came a 27-17 defeat in Denver that ruined the Chiefs’ perfect season, and then came a length-of-the-field drive by Rivers in the final minute to give San Diego a 41-38 victory in a game that Kansas City thought it had won after a late touchdown of its own.

On Sunday, the Broncos scored 28 straight points spanning halftime to seize control, and then held on when Alex Smith’s pass on fourth down into the end zone with 1:46 left fell incomplete.

“We just have to play ball,” Chiefs safety Eric Berry said. “Nobody can have their head down. It was a tough game and they happen when you play against a good football team. We just have to go in with a sense of urgency and get some things done.”

They might want to start with reassessing their defense.

A team that was once on pace to set an NFL record for sacks in a season with 35 through the first seven games has gotten to the quarterback just twice in the last five games. That lack of pressure has exposed a secondary that has been torched by Manning and Rivers the past few weeks.

The Chiefs allowed an average of 208.3 pass yards and 326.9 total yards during their nine-game win streak, and the result was 13.3 points per game. They’re allowing an average of 371 pass yards and 484.3 total yards during their skid, and 34.3 points per game.

“You know, earlier in the year we were coming out the other end of it,” Smith said. “We just have to continue to fight. I think this team has that kind of character.”

For all its recent ills, Kansas City can still clinch a playoff berth Sunday with a win over downtrodden Washington and either a Miami loss at Pittsburgh or a Baltimore loss to Minnesota.

“We still have everything we want in front of us,” Smith said. “It’s no time to point fingers or do anything like that. Just continue to do what we’ve been doing.”